Spark plugs are used in combustion chambers of a combustion system such as an internal combustion engine to ignite fuel mixture. In general, a spark plug includes an electrode-attached insulator having an insulator that is made of a ceramic having insulation properties. The insulator is provided with an axial hole which extends in an axial direction and a center electrode that is inserted in the axial hole to be positioned on a front-end side of the axial hole such that a front end portion is exposed from a front end of the insulator. Additionally, the spark plug includes a cylindrical metal shell that is provided on an outer circumference of the electrode-attached insulator. Aground electrode is fixed to the metal shell and defines a gap between the center electrode and itself. Then, a predetermined voltage is applied to the gap to generate a spark discharge in the gap, whereby fuel mixture is ignited.
In general, the center electrode is inserted into the axial hole from a rear-end side opening and is held in place therein by a collar portion formed thereon at a rear-end side so as to protrude radially outwards, the collar portion being locked on a step portion provided on an inner circumference of the axial hole. Here, an outside diameter of a portion of the center electrode which is exposed from the front end of the insulator is made smaller than a minimum bore diameter of the axial hole so that the front end portion of the center electrode is exposed from the front end of the insulator.
Incidentally, the central electrode wears in association with generation of spark discharge, and the size of the gap is gradually increased. This increases gradually the voltage necessary to generate spark discharge (the discharge voltage), and when the discharge voltage is increased excessively, spark discharge cannot be generated (generally referred to as a “misfire” is called for).
Then, in order to realize an increase in durability, it is considered to increase the outside diameter of the portion of the central electrode which is exposed, i.e., extends, from the distal end of the insulator (the portion of the central electrode which define the gap). The outside diameter of the central electrode is increased so as to increase the volume of the central electrode to be worn until a misfire takes place. As an approach to increase the volume of the portion of the center electrode which is exposed from the insulator, an approach is considered in which an annular ring member is joined to an outer circumference of the front end portion of the center electrode after the center electrode has been inserted through the insulator (for example, refer to JP-A-2002-141154).
In this approach, however, since the ring member is joined to the center electrode after the center electrode has been inserted through the insulator, and the center electrode is inclined when the ring member is joined to the center electrode, a radial axis deviation tends to easily occur between the axis and a center line of the center electrode. Once the axis deviation occurs, there are concerns that a gap of a predetermined size cannot be formed any more or abnormal discharge creeping over the surface of the insulator (a so-called flashover) is easily generated between the center electrode and the metal shell.
In contrast to this, it is considered to increase the thickness of the center electrode so as to increase the volume of the portion of the center electrode which is exposed from the front end of the insulator. In this case, however, the minimum bore diameter of the axial hole needs to be increased to deal with the increase in the outside diameter of the center electrode. Because of this, the insulator has to be made thin, causing fears that a reduction in the withstand voltage performance of the insulator is called for.